The increasing miniaturization and the required flexibility of mobile radio systems make it necessary to design power amplifiers for the transmission path of a mobile radio device as integrated circuits in a semiconductor body with increasingly smaller dimensions. Moreover, modern mobile radio devices are embodied such that they operate with a plurality of different mobile radio standards which transmit their signals at different frequencies. One example thereof is devices for the WLAN standards 802.11a,b and 802.11g, the frequency bands of which lie in the region of 2.4 GHz and 5.1 GHz. At different frequencies, it may become necessary to provide a plurality of different power amplifiers for the individual frequency bands. However, such a solution is not expedient for cost and space reasons.
One alternative to this is an integrated power amplifier having a plurality of amplifier sections arranged in parallel, each individual amplifier section being optimized for a specific frequency band. In this case, the signal to be amplified is applied to different inputs that are assigned to the amplifier sections. The outputs of the amplifier are connected to a common or to different antennas by suitable means. One example of a power amplifier is the PMB 8825 amplifier from Infineon Technologies AG. This amplifier is designed principally for amplifying signals according to the Wireless LAN mobile radio standard. The standard uses the freely available frequency bands (ISM bands) in the range of 2.4 GHz to 2.5 GHz and also 5.15 GHz to 5.825 GHz. The PMB 8825 power amplifier is based on Si—Ge technology and has in each case dedicated input terminals and output taps for the individual frequency bands.
Particularly in the region of the two frequency bands for the Wireless LAN standards IEEE 802.11a, 11b and 11g it is necessary to ensure sufficient adjacent channel rejection. What is problematic in the case of power amplifiers having a plurality of parallel amplifier types for individual frequency bands is crosstalk of harmonic frequency components of a signal into another amplifier section. By way of example, crosstalk is readily possible precisely in the case of amplifiers for the frequency bands 2.4 GHz to 2.5 GHz and 5.15 GHz to 5.825 GHz. Crosstalk is effected inter alia when, in the case of an output signal of 2.4 GHz, the signal component of the second harmonic of the output signal at 2.4 GHz couples over to the 5 GHz output of the power amplifier. Harmonic components or harmonics for short represent signals whose frequency corresponds to integral multiples of the frequency of the actual useful signal.
Important harmonic components are the second harmonic at double the useful signal frequency, and the third harmonic at triple the frequency. Harmonic components are generated by nonlinear behavior in active components of the amplifier, for example in the power transistors. Crosstalk of the harmonics into the parallel-arranged amplifier sections of other frequency ranges necessitates complicated shielding measures within and outside the chip.